Present day lifting or lowering systems for blinds or shades utilize one or more lift cords in order to raise or lower the blind or shade. The lift cord usually wraps around a movable shaft, which is driven by a drive shaft. Ideally, the lift cord should be wound around the movable shaft in a single layer, preferably with the lift cord wound with adjacent abutting loops, allowing the lift cord to raise or lower the blind or shade without any difficulty.
Mechanisms have been devised which attempt to have the lift cord wrap around the movable shaft in a single layer in an adjacent fashion. For example, U.S. Pat. No. 5,328,113 to de Chevron Villette discloses a drum which is attached to the lift cord. The lift cord initially is wrapped around a cylindrical or conical drum of greater diameter. As the cord advances along the cylindrical or conical drum, the cord eventually falls off this drum portion with limited tension, thereby enabling the cord to wrap around the lift cord drum in a single layer.
Other mechanisms which attempt to have the lift cord wrap around the movable shaft in a single layer are exemplified by the patent to Domel, U.S. Pat. No. 5,725,040, and the patent to Colson, U.S. Pat. No. 6,223,802. Domel discloses a relatively long spindle channel and spindle, with the distance between the two approximating the diameter of the suspension cord. Colson discloses an outer cylindrical shell distanced away from the spring spool by a distance slightly greater than the diameter of the lift cord and extending the entire length of the shaft.
Despite efforts to have the lift cord wrap around the winding drum or shaft in a single layer, tangling of the lift cord often results when only one end of the blind or shade is lifted, or if one end of the blind or shade hits an obstruction as the blind or shade is raised or lowered. The lift cord thus becomes tangled along the movable shaft onto which the cord normally is wrapped. This tangling, often called “bird nesting,” is particularly troublesome if the lift cord becomes tangled over the drive shaft. Once bird-nesting occurs over the drive shaft, it is often difficult to restore the cord back onto its desired position on the movable shaft.
Neither the mechanisms disclosed in the Domel or the Carlson patents remedy this situation. In fact, because the lift cord is constrained within a small space only slightly greater than the diameter of the lift cord itself, the cord can easily jam within this space thereby making proper operation problematic.